1. Field of the Invention
The present invention relates in general to an apparatus for controlling a motor vehicle such that a fuel supply to an engine is cut while a lock-up clutch in a power transmitting device is engaged when a throttle valve of the engine is placed in its engine idling position. More particularly, this invention is concerned with a technique for controlling the lock-up clutch while the throttle valve is in an alternate idling and non-idling state as in an automatic cruising mode wherein the amount of opening of the throttle valve is automatically controlled so as to enable the vehicle to run at a substantially constant speed.
2. Discussion of the Related Art
There is known a motor vehicle equipped with (a) fuel cutting means for cutting a fuel supply to an engine when a throttle valve for controlling an intake air quantity of the engine is placed in its engine idling position, and (b) clutch control means for effecting partial or full engagement of a lock-up clutch while the throttle valve is placed in the engine idling position. The lock-up clutch is disposed in parallel with a torque converter, fluid coupling or other fluid-filled power transmitting device adapted to transmit an output of the engine to drive wheels of the vehicle. An example of a motor vehicle control apparatus equipped with such fuel cutting means and clutch control means is disclosed in JP-A-5-149423.
The fuel cutting means is actuated while the throttle valve is almost fully closed or placed in its engine idling position. Usually, the fuel cutting means is activated while the vehicle is in deceleration with the throttle valve placed in the engine idling position, for the purpose of improving the fuel economy of the vehicle and preventing overheating of a catalyst used for the engine. Generally, the fuel cut is initiated when the engine speed is higher than a predetermined fuel-cut threshold, and is terminated when the engine speed falls below a predetermined fuel-supply threshold. On the other hand, the clutch control means is provided to control the lock-up clutch for preventing a rapid drop of the engine speed to thereby increase the time duration of the fuel cut, that is, to increase a range of the vehicle speed within which the fuel cut is continued. Usually, the clutch engagement means is kept in operation until the fuel cutting means is turned off, that is, until the fuel supply is resumed.
Referring to graphs of FIGS. 10A and 10B, there are shown examples of relationships between a speed N.sub.E of the engine, and a speed N.sub.T of a turbine impeller in a power transmission system equipped with a fluid-filled power transmitting device in the form of a torque converter, when the throttle valve is placed in the engine idling position. The graph in FIG. 10A indicates the relationship between the engine speed N.sub.E and the turbine impeller speed N.sub.T when the amount of slip (the degree of partial engagement) of the lock-up clutch is controlled by the clutch control means, while the graph of FIG. 10B indicates the relationship when the lock-up clutch is not controlled by the clutch control means. The fuel cutting means is turned on to initiate the fuel cut of the engine if the throttle valve is in the engine idling position while the engine speed N.sub.E is higher than a predetermined fuel-cut threshold N.sub.ON. When the engine speed N.sub.E drops below a predetermined fuel-supply threshold N.sub.OFF as a result of the fuel cut, the fuel cut is terminated. As is apparent from the graphs, the vehicle speed V when the engine speed N.sub.E has been lowered to the fuel-supply threshold N.sub.OFF is lower in the case of FIG. 10A in which the lock-up clutch is controlled, than in the case of FIG. 10B. Accordingly, the fuel cut is effected over a wider range of the vehicle speed V when the lock-up clutch is controlled in the slip control mode, than when the lock-up clutch is not controlled.
There is also known a motor vehicle equipped with (c) automatic cruising control means for automatically controlling the amount of opening of the throttle valve so that the vehicle runs at a substantially constant speed. Various methods have been proposed for effecting the automatic cruising control. According to one proposed method, the actual vehicle speed is compared with upper and lower limits which are determined depending upon a desired vehicle speed (hereinafter referred to as "auto-cruising speed" when appropriate) preset by a vehicle operator, and the opening of the throttle valve is reduced by a predetermined amount when the actual vehicle speed rises above the upper limit, and is increased by a predetermined amount when the actual vehicle speed falls below the lower limit. According to an alternative method, the amount of opening of the throttle valve is controlled in a feedback manner, depending upon the amount of an error or difference between the actual vehicle speed and the preset auto-cruising speed.
In some cases where the vehicle is running on a downhill having a relatively low gradient, the overall running resistance of the vehicle may be zero, and the vehicle speed can be maintained substantially constant with the throttle valve kept in the engine idling position. The overall running resistance includes an air resistance, a rolling resistance and a hill climbing resistance (negative value in the case of a downhill). If the automatic cruising control is effected in such condition, the throttle valve is placed in an alternate idling and non-idling state wherein the amount of opening of the throttle valve is alternately increased and reduced in a small degree near the engine idling position. Namely, the alternate idling and non-idling state is a state in which the throttle valve is alternately placed in the engine idling position and a position near the engine idling position, at a relatively high frequency. If the vehicle is equipped with the fuel cutting means and the clutch control means, the fuel supply to the engine is cut and the lock-up clutch is controlled with the throttle valve placed in the engine idling position, whereby the vehicle is subject to a larger engine braking effect than in the case where the fuel cut and the lock-up clutch control are not effected. This results in a higher rate of reduction of the vehicle speed. When the vehicle speed is reduced below the lower limit of the automatic cruising control, the amount of opening of the throttle valve is increased, and the fuel cut is terminated, whereby the vehicle speed is raised. The higher rate of reduction of the vehicle speed indicated above leads to hunting or periodic variation of the vehicle speed at a relatively high frequency, and a periodic variation of the drive torque at the same frequency due to alternate fuel cut and supply of the engine by the fuel cutting means. These hunting and variation are unexpected and undesirable to the vehicle operator. Where the lock-up clutch is controlled in the slip control mode in such condition, in particular, the fuel cut tends to be effected even at a relatively low vehicle speed, and the problem of hunting or variation of the vehicle speed and drive torque is accordingly aggravated.
The above problem is likely to occur during the automatic cruising control. However, a similar problem may occur when the vehicle operator manipulates the accelerator pedal so as to maintain the vehicle speed at a substantially constant level, since this manipulation has substantially the same effect as the automatic cruising control by the automatic cruising control means. The relatively high hunting frequency of the vehicle speed indicative above means a relatively high frequency at which the operating amount of the accelerator pedal should be increased and decreased by the vehicle operator. Since the engine braking force and the drive torque vary at a high frequency due to alternate idling and non-idling of the engine, the manipulation of the accelerator pedal so as to hold the vehicle speed substantially constant is difficult. The throttle valve may be placed in the alternate idling and non-idling state even when the accelerator pedal is manipulated by the vehicle operator so as to accelerate or decelerate the vehicle. In this case, too, the driving comfort of the vehicle as felt by the vehicle operator is deteriorated due to a relatively high frequency of hunting of the vehicle speed and drive torque, and the manipulation of the accelerator pedal tends to be difficult.